widgets like modals and dropdowns, often with an accompanying Javascript library

Many modern libraries also add support for CSS preprocessors like SASS, LESS and Stylus.

There are many, many competing CSS frameworks, that differ in both their look and approach. The biggest names at the moment (11/17) are Twitter Bootstrap, Semantic UI and Foundation. Personally, I find Tachyons interesting for their DRY approach.

Some CSS libraries only fulfill one of the above features, like the popular 960 grid system.

Why it matters

CSS libraries are massively popular because many developers hate wasting time writing CSS. Frameworks let you quickly mock up an app that will look reasonably good without running into the inevitable oddities and frustration of working with CSS. In theory, anyway.

The downside is that CSS libraries add bloat and slow down page load times, and cause your site to look the same as every other site using the framework. You know that bootstrap look.

Plus, adopting a CSS library can cause you pain down the road if you ever want to get rid of it: some compare using bootstrap to taking out a mortgage. Worse, many developers use CSS frameworks as a crutch that prevents them from needing to really understand CSS, leading to sloppy and broken code.

There’s a time and a place where CSS libraries make sense, but make sure you know CSS before you start relying on them.

Related guides

How to learn it

First, take the time to truly understand CSS. Then, do some research and pick a framework that you like. Often, the best way to learn to use it is by reading the official documentation. Alternatively, there are tutorials and classes out there for the most popular frameworks, such as: